securerandom.rb: fix an example of choose [ci skip]

* lib/securerandom.rb (Random::Formatter#choose): [DOC] fix an
  example, `n` is not optional.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60420 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/securerandom.rb

@@ -236,7 +236,7 @@ module Random::Formatter
   #
   # The result may contain whatever characters are in the source array.
   #
-  #   p SecureRandom.choose([*'l'..'r']) #=> "lmrqpoonmmlqlron"
+  #   p SecureRandom.choose([*'l'..'r'], 16) #=> "lmrqpoonmmlqlron"
   #   p SecureRandom.choose([*'0'..'9'], 5) #=> "27309"
   #
   # If a secure random number generator is not available,